Approximately 40,000 very low birth weight ("VLBW") infants (less than 1,500 gm) are born in the United States each year. Ventura et al., "Advance report of final natality statistics, 1994." Monthly Vital Statistics Report 1996; 44:1-88. Survival of this group has improved with advances in neonatal intensive care, but late-onset sepsis and necrotizing enterocolitis ("NEC") continue to be major causes of morbidity and mortality. Stoll B J, Gordon T, Korones S B, Shankaran S, Tyson J E, Bauer C R, "Late-onset sepsis in very low birth weight neonates: a report from the National Institute of Child Health and Human Development Neonatal Research Network." Journal of Pediatrics 1996; 129:63-71. Gray J E, Richardson D K, McCormick M C, Goldmann D A, "Coagulase-negative staphylococcal bacteremia among very low birth weight infants: relation to admission illness severity, resource use, and outcome." Pediatrics 1995; 95:225-230. Unfortunately these illnesses are common in neonates, and infected infants have a significant increase in the number of days spent on the ventilator and an average increase in duration of hospital stay of 25 days. See Stoll et al. above.
Neonatal sepsis occurs in 5 to 15% of infants weighing less than 1,500 gm at birth, and the rate is about 1 per 100 patient days. Gladstone, I. M., R. A. Ehrenkrantz, S. C. Edberg, and R. S. Baltimore. 1990. "A ten-year review of neonatal sepsis and comparison with the previous fifty year experience." Pediatric Infectious Disease Journal 9:819-825. Moro, M. L., A. DeToni, I. Stolfi, M. P. Carrieri, M. Braga, and C. Zunin. 1996. "Risk factors for nosocomial sepsis in newborn infants and intermediate care units." European Journal of Pediatrics 155:315-322. The National Institute of Child Health & Human Development ("NICHID") Neonatal Research Network found that neonates who develop late-onset sepsis have a 17% mortality rate, more than twice the 7% mortality rate of noninfected infants.
Risk factors for late-onset sepsis are ubiquitous in the NICU: intubation, umbilical catheters, prolonged mechanical ventilation, low birth weight, parenteral nutrition via central venous catheters, respiratory distress syndrome, bronchopulmonary dysplasia, severe intraventricular hemorrhage, and nasogastric and tracheal cannulae are all independently associated with sepsis. See Moro et al. supra. Each interventional device represents a potential source of infection and increases the risk of catastrophic infectious illness. Id.
Necrotizing enterocolitis affects up to 4,000 infants in the U.S. yearly, and an estimated 10 to 50% of infants who develop NEC die. Neu, J. 1996. "Necrotizing enterocolitis." Pediatric Clinics of North America 43:409-432. Infants who develop NEC often require intubation and an increase in respiratory support. Survivors are often left with strictures and short-bowel syndrome.
Unfortunately, prior to the discovery of the present invention there has been no reliable clinical means for early diagnosis of these diseases. Clinical neonatologists caring for these VLBW infants recognize sepsis and NEC as potentially catastrophic illnesses, and thus do not hesitate to obtain blood cultures and to administer antibiotics empirically at the first appearance of symptoms in an attempt to avert disaster. Likewise, physicians do not hesitate to stop feeding and obtain radiographic studies should any abdominal finding occur. Unfortunately, clinical signs are neither sensitive nor specific in these illnesses, resulting in many unnecessary blood cultures, unnecessary administration of short courses of antibiotics given to infants without bacterial infection, and unnecessary interruptions in neonatal nutrition. Moreover, despite these practices, sepsis and necrotizing enterocolitis continue to occur and continue to cause neonatal deaths. Indeed, by the time clinical signs and symptoms for either sepsis or NEC have developed, the illness may have progressed to an irreversible stage. Thus a successful surveillance strategy which leads to an earlier diagnosis of sepsis and NEC for the VLBW infants is necessary and critical in decreasing mortality and morbidity.
In healthy newborn infants, time series of heart period (or RR intervals, the time between successive heart beats) show obvious variability. Numerous publications are available which detail the measurement and characterization of such heart rate variability (HRV). See, e.g., 1. Ori, Z., G. Monir, J. Weiss, X. Sayhouni, and D. H. Singer. 1992. "Heart rate variability: frequency domain analysis." Cardiology Clinics 10:499-533. Kleiger, R. E., P. K. Stein, M. S. Bosner, and J. N. Rottman. 1992. "Time domain measurements of heart rate variability." Cardiology Clinics 10:487-498. HRV arises from the interplay of the sympathetic and parasympathetic arms of the autonomic nervous system, which act respectively to speed or slow the heart rate. In newborn infants, as in adults, HRV is substantially reduced during severe illness. Burnard, E. D. 1959. "Changes in heart size in the dyspnoeic newborn infant." Brit Med J 1:1495-1500. Rudolph, A. J., C. Vallbona, and M. M. Desmond. 1965. "Cardiodynamic studies in the newborn. III. Heart rate patterns in infants with idiopathic respiratory distress syndrome." Pediatrics 36:551-559. Cabal, L. A., B. Siassi, B. Zanini, J. E. Hodgman, and E. E. Hon. 1980. "Factors affecting heart rate variability in preterm infants." Pediatrics 65:50-56. Griffin, M. P., D. F. Scollan, and J. R. Moorman. 1994. "The dynamic range of neonatal heart rate variability." J.Cardiovasc.Electrophysiol. 5:112-124.
The reasons for reduced HRV during illness has been debated, and three theories concerning the mechanisms of reduced HRV have been developed. These theories focus on the mathematical characteristics of RR interval time series showing normal and low HRV.
The first theory focuses on the notion that the mechanism behind reduced HRV is a reduction of parasympathetic tone. Akselrod, S., D. Gordon, F. A. Ubel, D. C. Shannon, A. C. Barger, and R. J. Cohen. 1981. "Power spectrum analysis of heart rate fluctuation: a quantitative probe of beat-to-beat cardiovascular control." Science 213:220-222. But see Malik, M. and A. J. Camm. 1993. "Heart rate variability: from facts to fancies." J Am Coll Cardiol 22:566-568. The second theory centers on the notion that normal physiology is more complex than abnormal, hence heart rhythm is more irregular during health. Goldberger, A. L., D. R. Rigney, and B. J. West. 1990. "Chaos and fractals in human physiology." Scientific American 262:42-46. Goldberger, A. L., V. Bhargava, B. J. West, and A. J. Mandell. 1985. "On a mechanism of cardiac electrical stability: the fractal hypothesis." Biophys J 48:525-528. Goldberger, A. L. and B. J. West. 1987. "Chaos in physiology: health or disease? In Chaos in biological systems." H. Degn, A. V. Holden, and L. F. Olsen, editors. Plenum Press, New York. 1-4. Goldberger, A. L. and B. J. West. 1987. "Applications of nonlinear dynamics to clinical cardiology." Ann NYAcad Sci 504:195-213. Goldberger, A. 1990. "Fractal electrodynamics of the heartbeat. In Mathematical Approaches to Cardiac Arrhythmias." J. Jalife, editor. The New York Academy of Sciences, New York. 402-409. Peng, C.-K., J. Mietus, J. M. Hausdorff, S. Havlin, H. E. Stanley, and A. L. Goldberger. 1993. "Long-range anticorrelations and non-Gaussian behavior of the heartbeat." Phys Rev Lett 70:1343-1346.
Without wishing to be held to any particular explanation or theory, we have developed a third theory of the mechanism of the observed abnormalities of HRV: an explanation based on the events of signal transduction cascades (Nelson J C, Rizwan-uddin, Griffin M P, Moorman J R. Probing the order of neonatal heart rate variability. Pediatric Research, 43: 823-831, 1998). The sinus node cell membrane has beta-adrenergic receptors which, on binding agonists released from sympathetic nerve endings or the adrenal medulla, lead to the activation of cAMP-dependent protein kinase, which phosphorylates cardiac ion channels and results in cell depolarization, an action potential, and a heartbeat. This readily explains the rise in heart rate after sympathetic stimulation. The sinus node cell membrane also contains muscarinic acetylcholine receptors--when bound with acetylcholine from parasympathetic nerve endings, the process is inhibited and the heart rate falls. As the amounts of sympathetic and parasympathetic activity vary, so heart rate varies. Thus, for as long as the complex steps of intracellular signal transduction can be successfully completed, we can view the sinus node as an amplifier of the input signals of the autonomic nervous system, and heart rate as the output signal.
Consider now a severe illness such as sepsis. In such an unfavorable metabolic milieu, optimal conditions for signal transduction are unlikely. We hypothesize that HRV becomes abnormal here because sinus node cells, like all other cells, are unable to respond normally to sympathetic and parasympathetic inputs. From this viewpoint, sinus node cells report in real time on their ability to respond to adrenergic and muscarinic stimuli. Effective reporting depends on optimal intracellular conditions, and we view HRV as a sensitive measure of the state of cells.
We thus hypothesized that monitoring HRV in patient populations at high risk leads to an early diagnosis and opportunity for early treatment for severe infections. We have found this to be the case. In particular, we have found that records of RR intervals in infants prior to the clinical diagnosis of sepsis demonstrate at least two characteristic abnormalities. First, the baseline shows very reduced variability. Second, there are short-lived episodes of deceleration of heart rate. We have developed novel mathematical approaches to detecting these characteristic abnormalities.
Heretofore, heart rate variability measurement has been used as a means of assigning long-term prognosis, usually in adults with heart disease. These measurements, however, typically involve only a single measurement of HRV rather than the continuous monitoring we describe.
Heart rate variability (HRV) is abnormal during neonatal illness, but the value of monitoring HRV as a means of early diagnosis of sepsis and necrotizing enterocolitis in premature neonates has not heretofore been tested. Conventional measures of HRV fail to detect the abnormal HRV in the infants because these measurements, such as standard deviation and power are optimized to detect low variability. Additionally, prior studies showing low HRV in newborn infants with severe illness have typically focused on term rather than premature infants. See, e.g., Griffin M P, Scollan D F, Moorman J R. "The dynamic range of neonatal heart rate variability." Journal of Cardiovascular Electrophysiology 1994; 5:112-124.